Classifications

• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03F—AMPLIFIERS
  • H03F1/00—Details of amplifiers with only discharge tubes, only semiconductor devices or only unspecified devices as amplifying elements
  • H03F1/32—Modifications of amplifiers to reduce non-linear distortion
  • H03F1/3223—Modifications of amplifiers to reduce non-linear distortion using feed-forward
  • H03F1/3229—Modifications of amplifiers to reduce non-linear distortion using feed-forward using a loop for error extraction and another loop for error subtraction
  • H03F1/3235—Modifications of amplifiers to reduce non-linear distortion using feed-forward using a loop for error extraction and another loop for error subtraction using a pilot signal
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03F—AMPLIFIERS
  • H03F1/00—Details of amplifiers with only discharge tubes, only semiconductor devices or only unspecified devices as amplifying elements
  • H03F1/32—Modifications of amplifiers to reduce non-linear distortion
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03F—AMPLIFIERS
  • H03F2201/00—Indexing scheme relating to details of amplifiers with only discharge tubes, only semiconductor devices or only unspecified devices as amplifying elements covered by H03F1/00
  • H03F2201/32—Indexing scheme relating to modifications of amplifiers to reduce non-linear distortion
  • H03F2201/3212—Using a control circuit to adjust amplitude and phase of a signal in a signal path

Definitions

• the present invention relates to amplifiers and in particular to high power amplifiers wherein feed forward cancellation is employed, such as those used in radio-frequency (r.f.) applications.
• a more efficient class AB amplifier may be employed if feed-forward cancellation is applied.
• the amplifier output (suitably scaled) is compared with the input signal to yield an error signal.
• the error signal is amplified and reintroduced to the output 180° out of phase with the original distortion, the distortion products being thereby cancelled in the final output.
• Feed forward can yield a 30dB improvement in distortion performance but only if both the first comparison loop and the second correcting loop are accurately aligned.
• the loop parameters do not remain constant over the full operating range of the amplifier and some dynamic correction is required, particularly in wideband application.
• the error signal may be subject to amplitude and phase control prior to amplification.
• a pilot tone may be injected into the amplifier input.
• the pilot signal (at residual distortion level) is detected at the output and used to provide control signals.
• an amplifier arrangement to which feed forward cancellation is applied by a comparison loop including comparison means for comparing amplifier input with amplifier output to provide an error signal, a cancellation loop including secondary amplifier means for amplifying the error signal and combining means for combining said amplified error signal with said amplifier output, a pilot generator coupled to said amplifier input to introduce a pilot tone therein, detector means for detecting a level of pilot tone in said amplifier output and correction means for correcting said cancellation loop performance as a function of said detection wherein said pilot generator is further coupled to a multiplier receiving said amplifier output, said multiplier producing an output signal arranged to control a loop parameter to effect said correction.
• the loop parameter is amplitude, or phase, or both.
• an amplifier arrangement in accordance with the present invention includes a further multiplier, said further multiplier producing an output signal arranged to control a further loop parameter.
• a parameter of the comparison loop for example phase.
• Figure 1 represents an amplifier having feed forward cancellation in accordance with the present invention
• Figure 2 represents an alternative embodiment of the present invention.
• Figure 3 represents a phasor diagram showing operation of the phase alignment loop.
• An r.f. high power amplifier 10 receives an input signal at its input 11 via a coupling 12 whenever a signal for amplification is applied to input port 14.
• the output of the amplifier 10 is tapped by a coupler 15 and fed to an input of a comparator 16, which may take the form of another directional coupler.
• the input signal applied at port 14 is fed to a second input of comparator 16 via a delay line 17, arranged to introduce a delay substantially equal to that of the power amplifier 10.
• Comparator 16 thus produces at its output an error signal representative of the difference between the input signal fed via amplified path a and delayed path b.
• the amplified path a includes an attenuator pad 18 to compensate for the signal gain introduced by the power amplifier 10.
• Paths a and b constitute the comparison loop of a feed forward arrangement.
• the error signal produced at the output of the comparator 16 is representative of distortion introduced by the power amplifier 10 and is fed via networks 19,100 to an error amplifier 101 and thence to an input of a combiner 102, which may take the form of a directional coupler.
• a second input of combiner 102 receives the output signal of the power amplifier 10 via a delay line 103 arranged to introduce a delay substantially equal to that introduced by comparator 16, networks 19 and 100 and error amplifier 101.
• Delay lines 17 and 103 may for example take the form of a length of coaxial cable transmission line.
• Combiner 102 serves to introduce the amplified error signal received via path c to the amplifier output signal received via path d such that the error signal is in anti-phase with the distortion present in the signal received via path d.
• the resultant signal produced at output port 104 thus has had feed forward distortion cancellation applied; paths c and d constituting the cancellation loop of the feed forward arrangement.
• An oscillator 105 is arranged to provide a pilot tone which is coupled into the power amplifier input 11 by a coupling 106. It should be noted that couplers 12 and 106 are arranged such that the oscillator signal is not coupled into input signal path b. This may be achieved by providing sufficient directivity from couplers 106 and 12. Since the pilot tone travels via path a only to comparator 16, it will contribute to the error signal and hence be cancelled by the cancellation loop. Thus the pilot tone will contribute distortion to the eventual output produced at port 104 that is no greater than the residual distortion that remains incorrected by the feed-forward loops.
• the oscillator feeds a first input of a multiplier 107 via a coupler 108, the second output of which is derived from the eventual output via a coupler 109.
• the output of the multiplier provides a maximum output when the residual pilot tone is in phase with the input signal from the oscillator.
• path 1 is not phase constant
• a phase work 24 is controlled in such a way that the path behaves as a constant phase path.
• delay network 200 is adjusted for maximum output from mixer 107.
• This output of mixer 107 is fed as a control signal to network 19, being a controllable attenuator used to adjust the amplitude of the error signal output of comparator 16 prior to amplification by error signal amplifier 101.
• network 19 is a controllable attenuator used to adjust the amplitude of the error signal output of comparator 16 prior to amplification by error signal amplifier 101.
• the mis-alignment produces a response of magnitude E at the output of mixer 107 which is amplified by a control amplifier 201 to give a signal of magnitude G C .E driving network 19, where G c is the gain of control amplifier 201.
• G c is the gain of control amplifier 201.
• the sign of G c has to be such that if E is produced by a lack of gain in amplifier 101, G C .E reduces the attenuation in network 19. It is desirable that a high gain amplifier 201 is provided.
• Phasor 34 represents the quadrature component of the unbalance signal which may be used as a basis for cancellation since for small 0, /sin 0 / (phasor 34) is approximately equal to / 0 / (phasor 33, to be corrected).
• Amplitude control network 19 may be a conventional PIN diode attenuator with dc control and phase control network 100 may take the form at a 90° coupler with a varicap diode controlled mismatched.
• control of the cancellation loop for minimum distortion allows better performance from the same power amplifier than with prior art feed forward techniques or alternatively an amplifier with a worse distortion performance (but better efficiency) may yield the same overall performance.
• an amplifier with a worse distortion performance but better efficiency
• the pilot tone may be selected to be conveniently within the frequency range of interest, control being wideband based on this representative performance. Alternatively two or more pilot tones may be introduced at different points across the band of interest, each controlling narrow band networks to permit a yet improved cancellation loop control.
• the performance of the arrangement described above may be improved by incorporation of a further feature of the invention, and such an embodiment will be described below.
• the amplitude applied to the mixers is preferably large, since the magnitude of the pilot tone is small (60dB down on amplified signal). If a signal of too large an amplitude is mixed, however, inter-moduction occurs and the loop provides no correction.
• the mixer input signals must be relatively low level, yielding a small dc output at the mixer for amplification by amplifier 201 to a level where control may be effected. Therefore the loop is dependant upon the mixer dc characteristics and it is generally the mixer dc stability that limits the overall performance of the loop.
• a low frequency oscillator 20 modulates the output of the pilot tone oscillator 105 via a mixer 21.
• Modulated outputs of mixers 107, 112 respectively representative of the in phase and quadrature components of the uncancelled pilot tone, are demodulated by envelope demodulators 22,23 respectively to yield dc control signals to control amplitude network 19 and phase network 100.
• modulation of the pilot tone means that ac outputs are available provided by multipliers 107 and 112 thus performance is not affected by multiplier dc offset or drift.
• mixers 107, 112 function as synchronous detectors, the envelope output being amplified by ac coupled amplifiers 25, 26 respectively.
• amplification is ac
• performance is not dependent upon the dc stability of the mixers.
• the envelope is detected by demodulators 22, 23, the dc outputs of which may be amplified by amplifiers 201, 28.
• the gain of these dc coupled amplifiers may be higher than the gain of the corresponding amplifiers in the arrangement since any drift or offset of the mixers is no longer coupled through. Hence the level of the pilot tone in the overall may be reduced.
• the ac amplifier may be arranged to contribute to the overall loop gain a proportion of the control gain thus being provided by the ac amplifiers.
• the present embodiment is particularly suitable for wideband applications. In such applications it is important that the phase shift of the pilot tone on its passage through the system is not so great that the phase margin of the negative feedback arrangement is destroyed.
• a phase control network 24 is provided in the comparison loop for this purpose.
• a pilot tone at 900 MHz was modulated by a low- frequency generator operating at lKHz.

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• Physics & Mathematics (AREA)
• Nonlinear Science (AREA)
• Engineering & Computer Science (AREA)
• Power Engineering (AREA)
• Amplifiers (AREA)
• Tone Control, Compression And Expansion, Limiting Amplitude (AREA)

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• 1989
  • 1989-11-16 GB GB8925959A patent/GB2238195A/en not_active Withdrawn
• 1990
  • 1990-11-12 EP EP90916266A patent/EP0453535A1/en not_active Withdrawn
  • 1990-11-12 WO PCT/EP1990/001890 patent/WO1991007812A1/en not_active Application Discontinuation
  • 1990-11-12 KR KR1019910700719A patent/KR920702078A/ko not_active Application Discontinuation
  • 1990-11-12 US US07/690,927 patent/US5323119A/en not_active Expired - Lifetime
  • 1990-11-12 JP JP2515158A patent/JPH04504345A/ja active Pending

Non-Patent Citations (1)

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